1. Field of the Invention
This invention relates to tolerance ring assemblies, wherein a tolerance ring provides an interference fit between parts of an assembly, where a first part has a cylindrical portion located in a cylindrical bore of a second part. The invention particularly relates to assemblies having a tolerance ring that provides an interference fit between a cylindrical component such as a shaft or a bearing and a housing for the shaft.
2. Summary of the Related Art
Improved engineering techniques have resulted in the need for greater accuracy of machine parts, raising manufacturing costs. Very close tolerances are required where press fits, splines, pins or keyways are employed to transmit torque in applications such as pulleys, flywheels or driveshafts.
Tolerance rings may be used to provide an interference fit between parts required to transmit torque. Tolerance rings provide a low cost means of providing an interference fit between parts that may not be machined to exact dimensions. Tolerance rings have a number of other potential advantages, such as compensating for different linear coefficients of expansion between the parts, allowing rapid apparatus assembly, and durability.
A tolerance ring generally comprises a strip of resilient material, for example a metal such as spring steel, the ends of which are brought together to form a ring. A band of protrusions extend radially outwards from the ring, or radially inwards towards the centre of the ring. Usually, the protrusions are formations, possibly regular formations, such as corrugations, ridges or waves.
When the ring is located in the annular space between, for example, a shaft and a bore in a housing in which the shaft is located, the protrusions are compressed. Each protrusion acts as a spring and exerts a radial force against the shaft and the surface of the bore, providing an interference fit between the shaft and the housing. Rotation of the housing or the shaft will produce similar rotation in the other of the shaft or the housing, as torque is transmitted by the tolerance ring. Typically, the band of protrusions is axially flanked by annular regions of the ring that have no formations (known in the art as “unformed regions” of the tolerance ring).
Although tolerance rings usually comprise a strip of resilient material that is curved to allow the easy formation of a ring by overlapping the ends of the strip, a tolerance ring may also be manufactured as an annular band. The term “tolerance ring” as used hereafter includes both types of tolerance ring. The term “shaft” as used hereafter includes any assembly component with a cylindrical portion, such as a shaft or a bearing.
Problems can occur during assembly of parts that use tolerance rings. As the tolerance ring requires a tight fit in the apparatus, there may be abrasion between the ring and various parts of the apparatus, which removes small fragments from the surface of the affected parts. These fragments are known in the art as particles. In particular, parts of the protrusions distal to the band of the ring may generate particles when sliding relative to part(s) of the apparatus. In certain apparatus, such as a computer hard disk drive where cleanliness is essential, production of particles is extremely undesirable, as the particles can adversely affect the function of the apparatus.
If the material of the housing (e.g. aluminium) is softer than the material of the shaft (e.g. steel), it may be desirable to have the protrusions slide relative to the shaft during assembly to minimise particle production. Normally, for this type of arrangement, the tolerance ring's protrusions point inwards.
For apparatus that includes a bearing, however, it may also be desirable that the bearing has the tolerance ring fitted thereto, and the protrusions engage the surface of the bearing in the housing i.e. the protrusions point outwards. The arrangement may be useful where the tolerance ring distributes torque from the housing to the bearing, as load is distributed evenly over a large contact area of the shaft provided by the tolerance ring. Where the surface area is not maximised, for example when the protrusions engage the bearing under load, there may be problems with an effect known as “torque ripple” where the torque in the bearing is not generated at a continuous level. It is preferable that load is distributed over as large an area as possible, for example over the base of the protrusions of the tolerance ring.
It may be advantageous to provide in the bore a tolerance ring with outward protrusions and fit the shaft into the ring. In this arrangement, although there may be reduced particle production as the protrusions do not slide relative to an apparatus component, particles may still be produced, as the end of the shaft may rub against the rim of the tolerance ring during assembly owing to the tight fit between the tolerance ring and the shaft.
Alternatively, it may be advantageous to fit a tolerance ring with inward protrusions to the shaft, and insert the shaft and tolerance ring into the bore. Although the inward protrusions do not rub against the surface of the bore, particles may be produced when the rim of the tolerance ring rubs against the edge of the bore in the housing, owing to the tight fit between the ring and the housing.
The need to reduce or eliminate particle production can therefore impose restrictions on the type of tolerance ring used. The tolerance ring that minimises particle production may not be the best choice for operation of the apparatus.